<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div>
    结果: <span></span>
</div>
<script>

let i = document.querySelector("#i1");
let e=document.querySelector("#i2");
let s = document.querySelector("span");
function f(x) {
    if (isNaN(i.value) || isNaN(e.value)) {
        s.innerText = "输入错误!";
        return;
    }
    switch (x) {
        case 1:
            s.innerText = i.value * 1 + e.value * 1;
            break;

        case 2:
            s.innerText = i.value - e.value;
            break;
        case 3:
            s.innerText = i.value * e.value;
            break;
        case 4:
            s.innerText = i.value / e.value;
            break;
    }
}
</script>
</body>
</html>